Resistors in Parallel and Resistors in Series

When working on addressable fire alarm systems or conventional fire alarm panels you will need to connect an end of line electrical resistor.  Each fire alarm control panel or addressable module type device will require a different value electrical resistor.  An end of line electrical resistor will only be required on class B wiring.  If you are installing a system in class A then the circuit will return back to the source and not require an end of line electrical resistor.   The most common resistor ohm values in fire alarm installation would include the following:

2.2K
3.9K
4.7K
10K
47K

The resistor ohm values can be found using a multi-meter of by the color codes printed on the end of line resistor itself.  For additional information on reading resistor color code, read this article. 

The concept behind end of line resistors is very simple.  The fire alarm panel and/or addressable fire alarm module type device puts out what we call supervisory voltage.  This voltage can either be reverse polarity or a lower voltage than that required to operate a specific field device.  The fire alarm panel or module is programmed to look for a specific resistance on that supervisory voltage.  If that value is not seen then the circuit will be treated as an open and sent into a trouble or alarm condition depending on the application.

With that said, it is crucial to install the fire alarm resistor at the end of line.  For fire alarm systems you will be required to install the resistors in parallel across the last device on the circuit.  This way, if any of the devices or wiring between the fire alarm panel and the last device become compromised then the circuit will show and open condition.  For intrusion systems place the resistor in series at the panel or last device.